Various signaling techniques or protocols have been used over the years to convey information over a network, such as a telephone communication network. For example, dual-tone multi-frequency (DTMF) signaling was developed (and is still used in various telephone communications networks today) as a signaling or communication method in telephone central offices, various branch exchanges and various other applications where signaling occurs over a line in a voice-frequency band.
Phone calls may be generated with cellular or mobile telephones in mobile communication networks. These telephones may use voice compression to generate a digital signal from an analog voice signal, e.g., by using common encoding techniques such as Code Excited Linear Prediction (CELP). CELP coding encodes a voice signal as a time-variable filter excited by a pulse stream at the so-called “pitch frequency” of a speaker. A consequence of this is that a voice signal that has a constant pitch frequency and relative low modulation for a duration similar to a DTMF digit duration (e.g., 45 ms) may have a line spectrum instead of a fairly broadband signal, for example in circumstances where the voice signal is not modulated. Tone lines of the generated line spectrum may be of uniform frequency spacing and may further correspond to a pair of frequencies of a DTMF tone pair, resulting in this voice signal mimicking machine generated DTMF signals. These circumstances necessitate discrimination between machine generated DTMF signals and voice simulated DTMF signals produced by encoding techniques.